Hot mix bituminous paving composition consists principally of aggregate and bitumen binder generally made by mixing pretreated aggregate and bitumen binder in either batch or continuous mixing equipment. Prior to mixing, the aggregate is heated to temperatures exceeding 150° C. to quantitatively remove both surface and pore-bound moisture. Bitumen is heated to temperatures typically exceeding 143° C. to lower the viscosity of the product and make it suitable for pumping through conventional liquid transfer equipment. The resulting paving composition typically has a temperature exceeding 143° C. upon exiting the mixing equipment. The high-temperature paving composition is typically referred to by those skilled in the art as hot-mix asphalt (“HMA”).
Elevated temperatures are needed in the production of HMA paving composition to ensure complete aggregate drying and adequate HMA fluidity to be easily processed through the hot-mix asphalt paving plant transfer equipment such as the buggy, belt, or slat conveyors used in silo storage systems. Additionally, the HMA paving composition is produced at temperatures exceeding 143° C. to ensure that it can be discharged uniformly from haul trucks; processed easily through asphalt paver equipment; and compacted to desired densities under compressive force of conventional, static, vibratory, or oscillatory steel and pneumatic compacting equipment.
Bitumen binders commonly used in fast-, medium-, and slow-setting bituminous paving applications are oil-in-water emulsions exhibiting viscosities and flow characteristics suitable for pumping, mixing, and spraying. For oil-in-water emulsions, the bitumen or oil phase is dispersed as stabilized droplets in a continuous phase of water.
For paving compositions used in the construction of load-bearing pavements, it is not practical to use an oil-in-water bitumen emulsion containing more than 75% by weight of bitumen based on total emulsion weight. The bitumen emulsion containing such high bitumen content has unacceptable handling and transfer properties. Additionally, raising temperatures to facilitate the handling is not an option because the water phase of such oil-in-water emulsion would evaporate resulting in further increases in viscosity.
In contrast to oil-in-water bitumen emulsion, water-in-oil bitumen dispersion is characterized as dispersed water droplets in a continuous oil phase of bitumen. The interface between the polar water and non-polar bitumen phases in the water-in-oil bitumen dispersion can be stabilized by the use of surface active agents. Typically, the dispersed water phase comprises less than 20% by weight based on the weight of water-in-oil bitumen dispersion.
U.S. Pat. No. 5,256,195 discloses a bitumen binder for bituminous paving composition consisting of a conventional bitumen emulsion of anionic or cationic type and a breaking additive comprising water-in-oil bitumen dispersion. The breaking additive is mixed into the conventional emulsion shortly before the use of the bitumen to control the breaking of bitumen emulsion in such a way that it is delayed but rapid once it has started. After a short delay time which is defined by the amount and composition of the additive, the bitumen emulsion breaks and develops rapidly a good ability to bind to stone material. The amount of breaking additive is generally about 1-15%, preferably 2-4%, by weight based on the weight of the finished bitumen emulsion. Water-in-oil dispersion used in the breaking additive comprises chiefly low-viscosity oils such as mineral oils and the like and/or low-viscosity bitumen such as bitumen flux. As a result, use of such water-in-oil dispersion breaking additive in paving composition reduces the early compressive strength of compacted pavement. Dosages of cutter stocks, as little as 0.1% by weight of the emulsion, often decrease the compacted pavement compressive strength until such time as the cutter stock has evaporated into the atmosphere. Decreased compressive strength may result in deformation under traffic; therefore, the emulsions containing such water-in-oil dispersion breaking additive are not suitable for use in the paving compositions for the construction of load-bearing pavements.
The bituminous paving compositions containing water-in-oil dispersion of paving grade bitumen (either performance-graded bitumens specified by the Strategic Highway Research Program, viscosity-graded bitumens, or penetration graded bitumens) exhibit viscosity that prevents flow at temperatures below the boiling point of water. Poor handling and transfer properties of such high viscosity dispersion prevent its use in the production of bituminous compositions for construction of load-bearing pavements utilizing conventional hot-mix asphalt production and construction equipment. Plant engineering controls and liquid transfer equipment are not compatible with use of such high-viscosity compositions. Water-in-oil bitumen dispersions made with conventional paving grade bitumen must be produced in a pressure vessel to prohibit an evaporation of water. Typically, paving grade bitumen must be heated to about 135° C. to ensure its sufficient fluidity for a proper processing in conventional colloid mill equipment. Combination of 80 parts bitumen at 135° C. with 20 parts water results in a water-in-oil bitumen dispersion having a temperature exceeding 135° C. Without backpressure, boiling of the water occurs.
To address the processing difficulty due to high viscosity, paving grade bitumen is typically pre-diluted with bitumen-compatible solvents such as diesel, naphtha, gasoline, kerosene, biodiesel, waste oils, and other suitable bitumen-compatible diluents. Pre-dilution of the bitumen reduces the required temperature of the bitumen phase during the production of water-in-oil dispersion, as well as prevents a potential boil out of the finished product. However, use of bitumen-compatible solvents or other diluents has undesirable consequences. Fugitive vapors in the solvent/diluent pose health hazards for worker and concerns for air pollutants due to the volatile emission. The solvent/diluent may leach into soils and groundwater supplies, deteriorating water and soil quality. Moreover, the solvent/diluent residue may remain in the bitumen of the finished pavement structure, causing a significant reduction in stiffness of the pavement. Reduction in stiffness, in turn, leads to deformation in the pavement structure under a load of traffic. Thus, application of such paving composition containing residue solvent/diluent is primarily limited to highways for rural and/or low traffic volume routes.
Hot mix bituminous paving compositions made of paving grade bitumen and designed for high-traffic load-bearing pavements, are normally produced by mixing the liquefied non-emulsified bitumen with preheated aggregate at elevated temperatures usually in excess of 150° C. Prior to mixing, the non-emulsified bitumen is liquefied by heating to temperatures far in excess of its melting point, and the aggregate is preheated in a rotating kiln at extremely high temperatures to drive off all water adsorbed within thereof. The finished hot mix paving composition containing aggregate and bitumen binder must be substantially free of water to ensure that the paving composition shows no moisture sensitivity once it is transported, laid down and compacted. Furthermore, the hot mix paving compositions must be produced, laid down, and compacted at the temperature in excess of 150° C., since its compactability depends on the temperature. The handling, placement and compaction of composition become extremely difficult and the design densities (air voids) cannot be achieved, if a temperature of the hot mix paving composition is below 100° C. Failure to reach the design densities results in deformation or rutting of the pavement layer in the wheel paths of vehicular traffic. Additionally, failure to reach design density may yield an overly porous pavement susceptible to moisture intrusion and moisture-related distress.
Therefore, in the construction of load-bearing pavements there is a need for a bituminous paving composition made of paving grade bitumen (either performance-graded bitumens specified by the Strategic Highway Research Program, viscosity-graded bitumens, or penetration graded bitumens) that can be produced using conventional production equipment, as well as easily processed and transferred.
Furthermore, in the construction of load-bearing pavements there is a need for a bituminous paving composition made of paving grade bitumen that is substantially free of volatile solvent/diluent to minimize, if not completely eliminate, an emission of volatile compounds and/or the amount volatile compound residues left in the finished pavement.
Additionally, in the construction of load-bearing pavements there is a need for bituminous paving composition that can be produced, transferred and applied at a lower temperature range than the typically high temperature required for hot-mix paving composition.
It is an object of the present invention to provide novel water-in-oil bitumen dispersions suitable for use in load-bearing pavements.
It is another object of the invention to provide bituminous paving compositions for load-bearing pavements containing water-in-oil bitumen dispersions of paving grade bitumen such as penetration-graded, viscosity-graded and/or penetration-graded varieties that have controllable and temperature-dependent interfacial rheology.
It is yet another object of the present invention to provide bituminous paving compositions for load-bearing pavements containing water-in-oil bitumen dispersions of paving grade bitumen such as penetration-graded, viscosity-graded and/or penetration-graded varieties that is substantially free of volatile solvents.
It is a further object of the invention to provide bituminous paving compositions for load-bearing pavements containing water-in-oil bitumen dispersions of paving grade bitumen, i.e. penetration-graded, viscosity-graded and/or penetration-graded varieties that can be produced using conventional production equipment such as in-line mixing methods involving static and/or dynamic mechanical unit operations in fixed and/or mobile asphalt mix plants of the batch, continuous, and/or dual varieties. The term “mobile” includes, but is not limited to, equipments used in-situ and in-place operations.
It is still a further object of the present invention is to provide bituminous paving compositions that exhibit substantially complete aggregate coating and compactability to required densities in the field, as well as rapidly develops load-bearing strength.
Other objects, features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.